Dimensionally stable stainless steel press plates and method of forming same



United States Patent 3,141,800 DIMENSHONA'LLY STABLE STAINLESS STEEL lgfiESS PLATES AND METHOD OF FORMING ME Albert T. Reichenbach, Washington Township, Westmoreland County, Pa., assignor to United States Steel Corporation, a corporation of New Jersey No Drawing. Filed Jan. 3, 1962, Ser. No. 164,181 2 Claims. (Cl. 148-125) This invention relates to stainless steel plates and, more particularly, to a method of producing and treating stainless steel press plates to be used under high pressures at elevated temperatures.

In the manufacture of laminated plastic sheets such as Formica the sheets are formed under high pressures at elevated temperatures. In a typical method of manufacturing the sheets, five smoothly finished stainless steel press plates are stacked on each other with resin impregnated sheets of paper interleaved between adjacent plates. The stack of plates with the impregnated paper is subjected to a pressure of about 1400 psi. and heated to about 300 F. This causes the impregnated paper between each of the adjacent plates to fuse into a hard plastic laminate. Pressure is then removed and the plates are cooled to about 100 F. at which point the formed plastic laminates are removed. The size of the plates used varies from about 30 inches x 60 inches to about 72 inches x 180 inches and from about .070 inch to .250 inch in thickness. Normally, these press plates are formed of a ferritic type stainless steel such as AISI Grade 430. Under cyclic loads and temperatures, plates formed according to the prior art have had a tendency to shrink during service. On some of the longer plates shrinking as much as 1 /2 inches has been recorded. When a plate shrinks to any appreciable extent it must be discarded since use of plates of unequal size will result in a substandard product due to fusing of the ends of the laminates and scoring of the plates known as cross breaks.

It is a principal object of this invention to provide dimensionally stable stainless steel press plates.

A related principal object of this invention is to provide a method of forming dimensionally stable stainless steel press plates.

Press plates made according to this invention are substantially free of shrinkage even under the cyclic pressure and temperature conditions described above.

In forming the press plates according to this invention, blanks are cut from a large sheet of material and are hot rolled until they are about 18% to 35 thicker than the final intended size. The hot rolled blanks are annealed between the recrystallization temperature and the lower critical temperature of the steel, preferably between 1350 F. and 1450 F. After annealing the blanks are pickled to remove the scale and then are cold reduced between 18% and 35% to the final press plate thickness. The cold reduction is followed by a second annealing also between the recrystallization temperature and the lower critical temperature of the steel, again preferably about 1350 F. to 1450 F. Following this annealing the sheets are again pickled for scale removal. At this point the sheets are flattened by stretch leveling, or roller leveling, or temper rolling, or combinations thereof, to produce a smooth flat plate. After the plates have been flattened it is necessary to stress relieve them, preferably between A registered trademark of the Formica Corporation.

3,141,800 Patented July 21, 1964 Ice 300 F. and 800 F. I have found that good results are obtained by stress relieving at about 500 F. The plates are then sheared to final size after which they are polished to a bright finish by grit. The plates are then ready for use.

An example of this method of forming press plates is as follows:

Blanks were cut from a sheet of 430 stainless steel and hot rolled at about 1650 F. to about 25% greater thickness than the final gage. The sheets were then box annealed at 1425 F. and soaked at this temperature for twelve hours. They were then pickled to remove the scale and were given a cold reduction of 25% to final gage of .109 inch.

After the cold reduction, the sheets were annealed in a continuous annealing furnace at 1425 F. being held at this temperature for two to three minutes. They were again pickled and then flattened by roller leveling, stretch leveling and temper rolling. The plates were stress relieved at 500 F. for twelve hours and then sheared to final size of 147 inches x 62 inches. Finally, the plates were polished and put into service.

It was found that the sheets formed according to this method exhibited exceptional performance with respect to dimensional stability and after repeated use showed no appreciable shrinkage.

Although the reason for such improved performance is not completely understood it has been determined that the final stress relief is critical. If this step is omitted shrinking problems are encountered. It is believed that stresses are induced into the plates by flattening, and after cyclic high pressure loading of the plates at elevated temperatures the stresses are released in a way which causes the plates to shrink. However, shrinkage during use can be avoided if these stresses are released prior to putting the sheets in service.

While one embodiment of my invention has been described it will be apparent that other adaptations and modifications may be made without departing from the scope of the appended claims.

I claim:

1. A method of producing dimensionally stable press plates of ferritic stainless steel comprising the steps of:

(a) shearing plate blanks from a sheet,

(b) hot rolling the blanks to a thickness greater than the final desired thickness,

(0) box annealing the blanks at a temperature above the recrystallization temperature and below the lower critical temperature of the steel,

(d) pickling the blanks,

(e) cold reducing the blanks to final press plate thickness,

(f) continuously annealing the plates above the recrystallization temperature and below the lower critical temperature of the steel,

(g) pickling the plates,

(h) flattening the plate,

(i) stress relieving the plates between 300 F. and

2. The method of claim 1 wherein the stress relieving temperature is about 500 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,905,577 Harris et al. Sept. 22, 1959 

1. A METHOD OF PRODUCING DIMENSIONALLY STABLE PRESS PLATES OF FERRITIC STAINLESS STEEL COMPRISING OF THE STEPS OF: (A) SHEARING PLATE BLANKS FROM A SHEET, (B) HOT ROLLING THE BLANKS TO A THICKNESS GREATER THAN THE FINAL DESIRED THICKNESS, (C) BOX ANNEALING THE BLANKS AT A TEMPEATURE ABOVE THE RECRYSTALLIZATION TEMPERATURE AND BELOW THE LOWER CRITICAL TEMPERATURE OF THE STEEL, (D) PICKLING THE BLANKS, (E) COLD REDUCING THE BLANKS TO FINAL PRESS PLATE THICKNESS, (F) CONTINUOUSLY ANNEALING THE PLATES ABOVE THE RECRYSTALLIZATION TEMPERATURE AND BELOW THE LOWER CRITICAL TEMPERATURE OF THE STEEL, (G) PICKLING THE PLATES, (H) FLATTENING THE PLATE, (I) STRESS RELIEVING THE PLATES BETWEEN 300*F. AND 800*F. 